The electrode tip has broken off a plug, and fallen into the spark plug bore.
One way to try and get it out would be to blow air into the cylinder, to try and dislodge the piece. The other way is to use a specialist tool with a bore scope and grab tool, to try and pick it out. If neither of these options work, then the head would need to be removed. I recommend that you have a mechanic, such as one from YourMechanic, come by and check your car for you, to see if they can get the piece out, without damaging the engine.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing